1. Field of the Invention
The invention relates to induction machine and other rotating equipment bearings and more particularly methods and apparatus for exerting preload bias forces on induction machine lubricated or active magnetic bearings that support rotating shafts, through use of passive, permanent magnet bearings.
2. Description of the Prior Art
Known rotating equipment and induction machines, such as motors, often utilize lubricated mechanical bearings to support a rotating rotor. Exemplary lubricated bearing types include rolling element anti-friction bearings (e.g., ball- or roller-type) wherein the rolling elements are lubricated by a non-pressurized boundary film layer between the element and its associated bearing race, hydrodynamic journal or thrust bearings that generate self supporting pressurized lubricant films and hydrostatic bearings that employ externally pressurized lubricant.
During motor or other rotating machine starting or stopping the rotor shaft may not be supported by a lubricating film within a lubricated bearing, possibly resulting in bearing rotational instability and/or premature bearing/shaft wear. One past solution has been application of auxiliary pressurized “oil jacking” systems to introduce pressurized lubricant to the bearings during induction machine starting or stopping cycles, analogous to a hydrostatic bearing. Such auxiliary systems add installation and maintenance expense to a machine installation, and may not be cost effective for smaller machines. In the past active magnetic bearings that require an electrical power supply to generate a levitation field have been used as replacements for or in conjunction with lubricated bearings in some high power output induction machines.
During normal motor operation, changes in driven shaft load or operating speed or lack of sufficient damping at the rotor shaft and bearing interface may cause lubricating film instability in the lubricated bearings. For example, rolling elements in anti-friction bearings may skid rather than roll relative to the corresponding bearing race, resulting in flat spots on the rolling element or bearing race scoring. In another example, oil slingers used to supply lubricant to bearings may fail to transfer sufficient quantities of lubricant if they lose contact with its corresponding rotor shaft journal. In another example, damping may be reduced below a useable threshold due to insufficient loading and/or high circumferential speed.
In the past active magnetic bearings have been used as the primary support bearings, in parallel with secondary support lubricated bearings in the event of magnetic bearing failure as in the case of a power loss. In these applications the active magnetic support bearings have exerted damping and or stiffening forces on spinning rotors that employ the secondary support lubricated bearings. However, the manufacture and operational costs and complexity of active bearings as compared to those of traditional lubricated bearings are not suitable for all induction machine applications. Additionally, systems which employ magnetic bearings must supply energy to the system to levitate the rotor against gravitational forces.
Thus, a need exists in the art for a method and apparatus that selectively apply desired oriented preload force direction and magnitude on induction machine lubricated bearings, in order to reduce machine wear during starting and stopping cycles.
Another need exists in the art for a method and apparatus that selectively apply desired oriented preload force direction and magnitude on induction machine lubricated bearings, in order to enhance bearing stability during machine operation, including transient machine operation, and in order to reduce bearing noise emission.
Yet another need exists in the art for a method and apparatus that passively and selectively apply desired oriented preload force direction and magnitude on induction machine lubricated bearings, without the need for external power sources and energy consumption associated with active magnetic bearings.
Yet another need exists in the art for a method and apparatus capable of reducing active magnetic bearing noise emission, and/or eliminate or reduce the need of additional lubricated bearings as secondary, back up bearings in the event of an active magnetic bearing failure.